The patent literature displays a wide range of proposals for smelting cells having improved performance, but none of these appear to be in current commercial operation, and most have some notable problems notwithstanding the claimed performance improvement.
In Payne U.S. Pat. No. 4,405,433 bubble release under the anode is described as being improved by the use of differential reactivity carbon. Improved bubble release by the use of steeply shaped anode/cathode sections is also outlined by Reynolds in their testwork. Improved resistivity performance was claimed by both but neither has been implemented on a commercial basis.
The patent literature also discloses the use of wettable materials (TIB.sub.2 based) which protrude from the metal pad as platforms or pedestals to yield an active cathode surface. These give a power reduction through reduced ACD but the effect is limited due to no gain in bubble release mechanisms at the anode. These types of cells have not been proven commercially viable, presumably because of a combination of material problems and the cost of construction. The cathode area available beneath the anode is also reduced compared to that of a flat metal pad when platforms or pedestals are used. In this type of cell the metal pad plays little role in carrying active current in the cell operations and is regarded as "non-active".
Another approach to minimizing ACD was that adopted by Seager (U.S. Pat. No. 3,492,208) who employed a wetted cathode material in a horizontal cell which was said to either continually drain into a sump region for collection for ease of tapping, or in which the metal pad was restricted to below 5 cm. Power savings were claimed to be achieved through the use of lower ACD's and due to the absence of magnetically driven movement of the metal pad experienced in conventional cells. However the trials described in the patent were only conducted at low amperage (10 kA) and no evidence was presented to indicate whether these conditions would hold at much higher amperage such as is now typically being used in the industry (80-300 Ka) and where electromagnetic disturbances of the metal are known to be a problem.
Boxall et al and others (e.g. U.S. Pat. No. 4,602,990) have adopted the use of angled drained cells to give both the benefits of low ACD operation and improved bath circulation by directional bubble release. With these cells bath circulation was considered critically important at low ACD operation. However, the bubble resistance problem remained.
Stedman et al (Australian Patent Application No. 50008/90 and U.S. Ser. No. 07/481847) have developed cells with improved performance by the use of a shaped cathode to induce shaping in the anodes to yield a anode having a double slope arrangement including a continuous longitudinal slope of the type envisaged by Boxall et al in U.S. Pat. No. 4,602,990, or having an induced bevelled section at its longitudinal edges.
Cells of this type have been trialled commercially but still suffer from some disadvantages in:
(i) increased construction complexity through the need for a large sump and for a special superstructure to hold sloping anodes. PA1 (ii) inefficient use of the anodes' carbon mass due to the angled profile not matching the horizontal surface of the bath, thus yielding anode rota problems.
These problems become more pronounced within larger cells using larger anodes, and produce difficulties in the ease of retrofit to existing plant conditions and/or work practices.